This invention relates generally to fluorescent lamps and more particularly to an ultraviolet fluorescent lamp assembly comprising a unique drive circuit and housing.
The fluorescent lamp is a gas discharge tube that is used for lighting purposes. Generally, the inner surface of the wall of the tube is coated with light-emitting substancesxe2x80x94usually fluorescent or phosphorescent metallic salts, and the tube is filled with mercury vapor at extremely low pressure and has filaments at each end of the tube. The light of the fluorescent lamp is not produced by an incandescent body (such as the filament of an ordinary electric lamp), but is emitted as a result of the excitation of atoms (namely, those of the mercury vapor and the fluorescent coating) and is extremely economical. The electrons ejected from the cathode filaments collide with the mercury atoms of the vapor and cause the mercury atoms to emit radiation which consists for the most part of ultraviolet rays, which are invisible. The ultraviolet light strikes the fluorescent substance with which the wall of the tube is coated and, depending upon the coating, may cause the substance to emit radiation with a longer wavelength in the visible rage of the spectrumxe2x80x94i.e., the coating may transform the invisible rays into visible light.
The conventional fluorescent lamp has to be operated with a choke (normally referred to as a ballast), which prevents a harmful rise in voltage and serves to ignite the lamp. For this purpose a starter circuit comprising a small auxiliary glow lamp provided with a thermal contact is usually connected in parallel with the main lamp. When the current is switched on, the glow lamp first lights up (the thermal contact is now open). This causes the contact to warm up and close, with the result that the glow lamp is short-circuited and the cathodes of the main lamp receive the full current. The thermal contact then cools and breaks, providing a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself. Because it is bypassed by the main lamp, the small auxiliary glow lamp then ceases to function.
Ultraviolet fluorescent lamps are often used in museum and other displays where powerful lighting is required to properly irradiate and display fluorescent mineral specimens. Fluorescent lamps are used with special ultraviolet filters that transmit the ultraviolet light and absorb the visible light that is generated by the lamps. However, most ultraviolet fluorescent lamps in use today have short useful life spans, and it is most commonly due to the inability of conventional drive circuits with conventional starter circuits to handle the high number of on-off cycles necessary in such a museum or other display. Another drawback of the currently available lamps is that no manufacturer of commercial ultraviolet lights uses a high output lamp.
Many prior inventors, such as Ewest and Yamamoto, have recognized the need to improve the starting and operating efficiency of fluorescent lamps. However, these prior art patents are directed simply to alternative designs of starter circuits. Ewest, in U.S. Pat. No. 1,961,749, discloses a gaseous electric discharge device which uses an auxiliary electrode in addition to the main electrode at one end of he lamp tube. This auxiliary electrode in proximity to the main electrode serves as a starter xe2x80x9cglow lampxe2x80x9d component for the main lamp. Ewest""s device also uses a high frequency apparatus to ionize the gas within the tube.
Yamamoto et al., in U.S. Pat. No. 5,107,183, disclose a fluorescent lamp which also uses a special arrangement of electrodes at one end or both ends of the lamp to constitute a starter xe2x80x9cglow lampxe2x80x9d component. Both Ewest""s device and Yamamoto""s device, because they use the same current supply to start the lamp as well as to maintain operation of the lamp, would have the same short lifespans of other extant devices.
One aspect of the present invention comprises a drive circuit for an ultraviolet (UV) lamp that does not use a conventional starter circuit.
The preferred embodiment of the drive circuit is designed around three unique custom-made lamps, and each circuit comprises at least one custom-made generally tubular lamp (sometimes called a bulb) that can be either short wave (made from a clear quartz tube that has a high UV-C transmission at 253.7 nm of about 90%), medium wave (made from a special erythemal glass with a UV-B phosphor on the inside of the lamp that will transmit the 306 nm wavelength), or long wave (made from a common soda-lime glass tube with a UV-A phosphor on the inside of the lamp that will transmit a peak output at 352 nm). Each lamp is a Rapid Start High Output lamp operating at the maximum amount of lamp current for that diameter and length of lamp. Each lamp has tungsten wire filaments on each end. In the making of the lamp the filaments are coated with an electron-emissive material (e.g., barium, strontium, and calcium as compounds) to turn them into lamp cathodes. The cathodes are designed for High Output lamp current. In addition there are wire or metal anodes on each cathode, the anodes helping prolong the life of the emissive material. Each lamp has a standard bi-pin base at each end, and in the preferred embodiment, all lamps are the same length (e.g., 22 xe2x85x9c inches from pin to pin) so that they are interchangeable within the entire assembly.
In addition to the lamp, the preferred embodiment of the drive circuit also comprises at least one each of a ballast subcircuit with ballast element and a transformer subcircuit with transformer element, and a lockout relay. The present invention of the drive circuit need not include the lockout relay; however, the relay is included in the preferred embodiment as an additional check upon the safe and correct operation of the drive circuit. The lockout relay prevents the high voltage of the ballast from being supplied to the lamp cathodes until the transformer subcircuit is powered and the lamp cathodes have been pre-heated.
The ballast element, which may be any type of appropriate ballast (such as an electronic ballast, but in the preferred embodiment is a conventional electromagnetic ballast), of the ballast subcircuit supplies high voltage (arc voltage) between the lamp cathodes at each end of the lamp. The ballast used in the preferred embodiment is a conventional electromagnetic ballast, but the filament windings are not used, and only one wire from each end of the ballast (high voltage) is connected to each end of the lamp. (Normally two wires from each end are connected to two pins of each end of the lamp.) For pre-heating the cathodes, the preferred embodiment of the drive circuit also comprises a separate transformer subcircuit which is used to supply low voltage (starter voltage) to the lamp cathodes, and which precludes the necessity of a conventional starter circuit. The transformer has two secondaries and four output leads, two per secondary, which lead to the lamp cathodes. In this way, cathode heat is supplied to each end of the lamp separate from the ballast. The transformer is always powered up before the high voltage from the ballast is supplied.
The average rated life of the lamp is greatly extended by having the filaments (cathodes) heated first before the high voltage (arc current) is applied. Heating the cathodes causes a space charge of electrons to form around the cathode filaments, the cloud of electrons helping to repel the heavy mercury (Hg) ions from impinging on the electron emissive material that is on the cathodes. Being repelled, the ions do not knock off emissive material that is on the cathodes. The lamps also have two anode wires on either side of the cathodes, which attract the ions away from the cathodes. This helps to prolong the life of the lamp, since the life of a fluorescent lamp is a function of the amount of emissive material that is on the cathodes. The resulting lamp life should approximate the continuous burning rated life of that particular lamp.
By having the lamp cathodes heated first, the high voltage from the ballast can be switched on and off thousands of times without effecting the life of the lamp. It also means that the output of the lamp is almost at full output when it is turned on and there is no xe2x80x9cflickeringxe2x80x9d or xe2x80x9cwarming upxe2x80x9d, especially if the lamp has had a few on-off cycles first. The advanced heating of the cathodes means that the lamp is effectively xe2x80x9cinstant startxe2x80x9d in application, i.e., the lamp will light to full output as soon as the ballast subcircuit is switched on, providing an economical operation, and no starters are required. Most other ultraviolet display lights have very reduced lives of their lamps because the lamps are turned on and off many thousands of times and often fail within a few thousand or maybe even a few hundred on-off cycles. In the preferred embodiment of the present invention, an on-off rocker switch is installed on the outside of the lamp housing assembly to turn on the high voltage of the ballast after the low voltage of the transformer has already been applied to the cathodes.
The lock-out relay of the preferred embodiment is a conventional electromagnetic relay situated between the two subcircuits of the ballast and the transformer. The lock-out relay ensures that the ballast subcircuit will not be completed unless there is current in the transformer subcircuit. In this way, the lamp cathodes are protected from inadvertently being xe2x80x9chitxe2x80x9d with the high voltage from the ballast unless they have been pre-heated by the transformer. This additional safety feature protects the circuit and prolongs the life of the lamp.
The lamp with its unique circuitry could be housed in any appropriate way that is fitting for the particular application. Obviously, there are other applications for such a fluorescent display than specifically for museum purposes.
However, another aspect of the present invention comprises a housing appropriate for use in a museum or other display case, in addition to the drive circuitry. This embodiment of the present invention includes a generally rectangular housing made of lightweight, coated aluminum comprising a box and cover. The box of the preferred embodiment is designed to house the lamp, lamp sockets, ballast, transformer, and lockout relay, as well as a terminal strip (in the preferred embodiment) and two safety fuses (one for the ballast subcircuit and one for the transformer subcircuit). The box of the preferred embodiment also houses a cooling fan, air dam, and reflector, and has a light baffle attached to the exterior of each end. The box of the preferred embodiment includes four protruding tabs on the sides of the box near the four corners. The housing can be easily installed in a display case by attaching wires or chains from the protruding tabs to the ceiling of the display area. Different lengths of wire or chain can be used to tilt the housing in the desired direction. Alternatively, the housing can be permanently attached to a display case through use of threaded nuts provided at appropriate locations on the top of the housing.
The housing is constructed so that only one hand-turned captive screw is required to secure the cover to the box. That one hand-turned screw makes it easy for a user to manually gain access to the interior of the box to replace the lamp while the housing is still mounted in the display case. Most other UV display lights have screws on several sides of the box and cover requiring a screwdriver or other tool for removal. Frequently, with other display light assemblies, the entire housing must be removed from the display case for a user to change the lamp or the filters.
The light assembly of the preferred embodiment has an ultraviolet filter located within the housing cover that is designed to transmit ultraviolet and absorb visible light that is generated by the lamp. A short wave filter may be used for short wave (SW), medium wave (MW), or long wave (LW) ultraviolet applications, while a long wave filter may be used for the long wave ultraviolet application. The combination of the custom-made lamp (SW, MW, or LW) and the correct filter makes the present invention useful for all ultraviolet wavelengths that are used in fluorescent mineral displays.
Once the cover is unscrewed by the hand-turned screw and is hanging free on its hinges, then the cover can be slid to the side and the cover will come off. The cover is attached by slip-hinges just for that purpose. Once the cover is removed it can be taken to a more suitable work area to replace the filters. The two identical filters are held in place by unique aluminum filter holders with xe2x80x9cfingers,xe2x80x9d which distribute the load evenly along the length of the filters. As the filters are generally quite expensive, two filters are used end-to-end so if one is accidentally broken, a larger whole filter will not have to be replaced. Short-wave ultraviolet filters necessarily solarize and so must be replaced periodically. (Obviously, only one large filter could be used, or any number of smaller filters.) The filter-holding xe2x80x9cfingersxe2x80x9d are intended to make a snug fit to hold the filters, but without putting so much pressure on the filters that they might crack the filter. When the filter holders are tightened, the xe2x80x9cfingersxe2x80x9d will xe2x80x9cgivexe2x80x9d some so that a more uniform pressure is applied to the filter edges without cracking the filter.
The preferred embodiment makes use of a terminal strip to organize all of the wires of the circuit. The terminal strip is not necessary to the invention, but in the preferred embodiment is located at one end of the interior of the box where the power cords enter the box and their component wires must be separated and routed to the appropriate subcircuit components. One safety fuse is provided for each power cord at the point where the power cord enters the box.
The fan helps to cool the lamp so that the mercury (Hg) vapor pressure in the lamp is kept relatively constant. If the Hg vapor pressure inside the lamp heats too much, the UV output will temporarily decrease. By maintaining a relatively constant airflow over the lamp, the Hg vapor pressure stays relatively constant and therefore the UV output stays constant. The high power of the custom-made lamps of the present invention is enhanced by maintaining an optimum temperature inside the lamp (i.e., other UV lamps lose output when heat in the lamps builds up). In the preferred embodiment, the fan is on any time the ballast subcircuit is powered; however, with different wiring configurations, the fan could be controlled by the transformer subcircuit or even by its own fan subcircuit.
The air dam of the preferred embodiment, although not necessary to the invention, is located at a convenient point between the reflector and the end wall and is intended not only to isolate the air within the electrical area, but also to force the airflow from the fan past the lamp tube in order to cool the gas in the tube.
Although there may be an optimum curvature to the reflector for maximum light output from the display case, in the preferred embodiment, the curvature chosen is a default for the configuration of the entire light assembly. The reflector of the preferred embodiment comprises a generally rectangular sheet of reflective coated aluminum which is curved around the tubular lamp and located within the box between the lamp and the drive circuit elements. The reflector has the effect of isolating the lamp from the other electrical components and elements in the housing and redistributing and directing all of the light from the lamp through the filters in the cover of the display case. The light baffles at either end of the box serve to reduce, if not entirely eliminate, the spillage of light from the interior of the box through any pathway but the filters in the opening in the cover.
The housing of the preferred embodiment also comprises several safety features. The electrical fuses have already been discussed. These fuses are normally located on the outside of one end of the housing so that they can be easily reached for maintenance. In addition to the fuses, there are warning labels on three sides of the housing stating that there should be a protective glass or plastic window between the short-wave or medium-wave light and any person""s eyes or skin. There also are, in the preferred embodiment, four rubber feet on the outside of the cover, which serve to protect the filters from breakage when the cover is being set down for any purpose.